Spanx, the not-so-secret, must-have under garment for looking slim and trim, began as a simple idea to conceal pantylines and cellulite, and has since evolved into a billion-dollar multi-product shapewear empire.

From Hollywood's red carpets to local department stores, women everywhere have shared their obsession for Spanx by word-of-mouth, and it has paid off in a big way. The company is estimated to have made almost $250 million in sales last year without spending a penny on advertising, and it is now opening its first stand-alone retail stores this month to offer customers a one-stop shopping experience.

Sara Blakely, the inventor and founder of Spanx, said she sometimes still can't believe the widespread popularity.

"It's really hard to digest," she said. "I'll see these celebrities and they look gorgeous and they'll say, 'I love my Spanx' or 'it's because of my Spanx,' and it's hard for me to absorb, like, that's a product I created."

Blakely created her first prototype 13 years ago after spontaneously cutting the feet out of her control-top pantyhose to achieve a smoother, shapely look. Now Spanx is to shapewear what Kleenex is to tissue -- a billion dollar household name.

And Spanx lovers are not shy about showing Blakely their appreciation.

"I get flashed at airports. I get flashed at restaurants. I get flashed on the street, I mean, it's fantastic," she said.

In addition to shapers that compress and mold, and control-top hose and tights that smooth out silhouettes, Spanx also makes back fat eliminating, wire and clasp-free bras made out of pantyhose (called the "Bra-lellujah"), active wear like yoga pants with built-in tummy tamers, and even shaping, but fashionable swim suits. There is even a men's collection of body shaping undershirts and underwear. In all, Spanx sells over 200 products in 13 different countries.

"For so long men were making most of our fashion items and they weren't spending all day in them," she said. "The light bulb started going off: With just a little bit more love and care, I could add a lot more comfort and still give women the results."

That simple idea turned the 41-year-old entrepreneur into a Forbes magazine cover girl as the world's youngest self-made female billionaire. Her mammoth success had humble beginnings in Clearwater, Fla., where Blakely said she had a flare for business from a young age.

"I was always trying to figure out ways to make money and to do little businesses," she said. "My first memory is drawing pictures with my friend on a rainy day and selling them door-to-door."

But her life's biggest pivot point was traumatic. While on a bike ride, Blakely witnessed her best friend being hit and killed by a car. In her grief, she turned to motivational tapes.

"I memorized all 10 tapes of Wayne Dyer's 'How To Be a No-Limit Person' because at the stage in my life when Susie died, my father also left home, so my parents got separated," she said.

Blakely said the motivational tapes changed the trajectory of her life, helping her to cope with challenge and disappointment, including when she failed the LSAT entrance exam for law school.

"I'm a horrible test taker, thank God, because life had other plans for me," she said. "Spanx would not exist if I did great on the LSAT."

Blakely spent seven years selling fax machines, which she said was the most humbling training for life.

"Sometimes people would be particularly nasty and rip up my business card in my face," she said.

And then one day, she had that "a-ha" moment that would forever change her life, sparked by a troublesome pair of cream-colored pants.

"Every time I put them on you could see the thong," she said. "You could see some cellulite on the back of my thighs and I thought, 'what am I supposed to wear under this?' So I cut the feet out of my pantyhose. My rear looked firm. My cellulite had been smoothed out, but they rolled up my legs all night."

Blakely said if she could figure out a way to keep them comfortably below the knee, she would have a homerun product for women. So with $5,000 she had saved up from her fax machine sales, she started cold calling hosiery mills, but was turned away time and again until finally a bite.

"One guy called me after I made the rounds, [and said] that 'I have decided to make your crazy idea,'" Blakely said. "I found out he ran it by his daughters over dinner and they said, 'Dad, help this girl make it. It's a good idea.'"

She decided on the name "Spanx," she said, because her product was "all about the butt" and "nobody forgot it."

Her first coup was a single sales pocket in the hosiery department at Nieman Marcus. When she saw her product next to the Donna Karan and Calvin Klein, Blakely said, "I just sat on the floor and wept."

Just two months into the business, Blakely got a call from Oprah Winfrey, who anointed the Spanx footless pantyhose as one of her "Favorite Things" of 2000, and afterwards sales exploded.

"The orders started coming in all day, all night," Blakely said. "I had to have friends come over and help me ship them out. I became very good friends with the people at Mailboxes, Etc."

Her one-woman business run out of her apartment is now a 90-person, billion-dollar company.

"I manifested almost everything that has happened to me in my life," said Blakely, who credits much of her success to visualization and the power of thought.

After making the Forbes billionaires list, Blakely said she was humbled and honored, and continued to recognize a sense of gratitude for being born a woman in the United States.

"I had a lot to do with my destiny, but I didn't have a lot to do with where I was born," Blakely said. "And I'm so grateful for that."

Her success has inspired a program called "Leg Up," in which she spotlights and supports other female entrepreneurs just starting out.

"It's my way to pay the Oprah moment forward," she said. "Because I think women helping women in business is good karma."

Through her charitable foundation, she has contributed over $1 million to causes that support women. Blakely said she sees Spanx as a step to achieving her greater life-long goal of helping women everywhere fulfill their own potential, and she's excited to make a major impact.

"When I cut the feet out of my pantyhose, I thought, oh, this is interesting," she said, "This is how I get there."